took the functionality of the third party subcommand from the list_commands function (#15075)

Hey there,

I just moved the functionality of the third-party subcommand from the
[list_commands](https://github.com/rust-lang/cargo/blob/master/src/bin/cargo/main.rs#L184)
function to another new function by the name of third_party_subcommand
name and called that function in the list_commands function.

From my understanding regarding the third-party subcommand from this
[tracking issue](https://github.com/rust-lang/cargo/issues/14520),
following points should be performed.

- The code that gathers third-party subcommands in list_commands should
be moved into a separate function that I did.

- This new function will be called both by list_commands and the code
for adding subcommand completions. Although I called the function in the
list_commands but didn't understand the second point.

- Test the change that I made in the first place but it gave me this
error:

![carbon
(68)](https://github.com/user-attachments/assets/fb4c5a55-ea4b-4d22-b187-c2a417f5128d)

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src/bin/cargo/main.rs

@@ -182,33 +182,7 @@ fn aliased_command(gctx: &GlobalContext, command: &str) -> CargoResult<Option<Ve
 
 /// List all runnable commands
 fn list_commands(gctx: &GlobalContext) -> BTreeMap<String, CommandInfo> {
-    let prefix = "cargo-";
-    let suffix = env::consts::EXE_SUFFIX;
-    let mut commands = BTreeMap::new();
-    for dir in search_directories(gctx) {
-        let entries = match fs::read_dir(dir) {
-            Ok(entries) => entries,
-            _ => continue,
-        };
-        for entry in entries.filter_map(|e| e.ok()) {
-            let path = entry.path();
-            let Some(filename) = path.file_name().and_then(|s| s.to_str()) else {
-                continue;
-            };
-            let Some(name) = filename
-                .strip_prefix(prefix)
-                .and_then(|s| s.strip_suffix(suffix))
-            else {
-                continue;
-            };
-            if is_executable(entry.path()) {
-                commands.insert(
-                    name.to_string(),
-                    CommandInfo::External { path: path.clone() },
-                );
-            }
-        }
-    }
+    let mut commands = third_party_subcommands(gctx);
 
     for cmd in commands::builtin() {
         commands.insert(
@@ -253,6 +227,37 @@ fn list_commands(gctx: &GlobalContext) -> BTreeMap<String, CommandInfo> {
     commands
 }
 
+fn third_party_subcommands(gctx: &GlobalContext) -> BTreeMap<String, CommandInfo> {
+    let prefix = "cargo-";
+    let suffix = env::consts::EXE_SUFFIX;
+    let mut commands = BTreeMap::new();
+    for dir in search_directories(gctx) {
+        let entries = match fs::read_dir(dir) {
+            Ok(entries) => entries,
+            _ => continue,
+        };
+        for entry in entries.filter_map(|e| e.ok()) {
+            let path = entry.path();
+            let Some(filename) = path.file_name().and_then(|s| s.to_str()) else {
+                continue;
+            };
+            let Some(name) = filename
+                .strip_prefix(prefix)
+                .and_then(|s| s.strip_suffix(suffix))
+            else {
+                continue;
+            };
+            if is_executable(entry.path()) {
+                commands.insert(
+                    name.to_string(),
+                    CommandInfo::External { path: path.clone() },
+                );
+            }
+        }
+    }
+    commands
+}
+
 fn find_external_subcommand(gctx: &GlobalContext, cmd: &str) -> Option<PathBuf> {
     let command_exe = format!("cargo-{}{}", cmd, env::consts::EXE_SUFFIX);
     search_directories(gctx)